Carbon Dioxide is a Powerful Inducer of Monokaryotic Hyphae and Spore Development in Cryptococcus gattii and Carbonic Anhydrase Activity is Dispensable in This Dimorphic Transition

Cryptococcus gattii is unique among human pathogenic fungi with specialized ecological niche on trees. Since leaves concentrate CO2, we investigated the role of this gaseous molecule in C. gattii biology and virulence. We focused on the genetic analyses of β-carbonic anhydrase (β-CA) encoded by C. gattii CAN1 and CAN2 as later is critical for CO2 sensing in a closely related pathogen C. neoformans. High CO2 conditions induced robust development of monokaryotic hyphae and spores in C. gattii. Conversely, high CO2 completely repressed hyphae development in sexual mating. Both CAN1 and CAN2 were dispensable for CO2 induced morphogenetic transitions. However, C. gattii CAN2 was essential for growth in ambient air similar to its reported role in C. neoformans. Both can1 and can2 mutants retained full pathogenic potential in vitro and in vivo. These results provide insight into C. gattii adaptation for arboreal growth and production of infectious propagules by β-CA independent mechanism(s).

格特隐球菌（Cryptococcus gattii）是一类以树木为特化生态位的人类致病真菌，在同类真菌中独具特色。由于植物叶片会富集二氧化碳（CO₂），本研究探讨了该气态分子在格特隐球菌的生物学过程与致病力中的作用。本研究重点针对格特隐球菌CAN1与CAN2基因编码的β碳酸酐酶（β-carbonic anhydrase, β-CA）开展遗传分析——鉴于CAN2在近缘致病菌新生隐球菌（C. neoformans）的二氧化碳感应过程中发挥关键功能。实验结果显示，高浓度二氧化碳条件可显著诱导格特隐球菌的单核菌丝与孢子旺盛发育；反之，高浓度二氧化碳则完全抑制其有性交配过程中的菌丝发育。CAN1与CAN2基因对于二氧化碳诱导的形态发生转换均非必需。然而，格特隐球菌的CAN2基因在常压空气环境中的生长不可或缺，这与其在新生隐球菌中的已报道功能一致。can1与can2基因缺失突变株在体外（in vitro）与体内（in vivo）实验中均保留了完整的致病潜能。本研究结果为理解格特隐球菌通过不依赖β碳酸酐酶的机制适应树栖生长并产生感染性繁殖体提供了新的见解。